docs: change the title of networking docs

The current title of our section of the documentation is
Linux Networking Documentation. Since we're describing
a section of Linux Documentation repeating those two
words seems redundant.

Link: https://lore.kernel.org/r/20220518234346.2088436-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>